Myopia, otherwise known as nearsightedness, is a common ailment. Myopia is a refractive defect of the eye in which images are focused within the vitreous inside of the eye rather than on the retina at the back of the eye, causing near object to appear in sharp resolution, but distant objects to appear blurred. A myopic condition is illustrated in FIG. 1. As shown, on-axis light rays 9 (light rays entering the eye 1 parallel to the central axis 7 of eye 1), and off-axis light rays 11 (light rays entering the eye at an angle of incidence relative to central axis 7 and refracted by the cornea 5 and crystalline lens 3 into eye 1), forming field of focus 13. In a myopic patient, field of focus 13 of axial light entering the eye is located in front of (anterior to) retina 15, causing decreased visual acuity. The curvature of the field of focus is in part determined by the curvature of anterior and posterior surfaces of the cornea, and thus varies from patient to patient.
Conventional myopia treatment techniques adjust the focal point of on-axis light rays by changing the refractive state of the cornea. As illustrated in FIG. 2, these techniques shift field of focus 13 back toward the retina 15, such that on-axis light rays 9 focus on fovea 16 located in the center of the retina 15. Conventional myopia treatment techniques include, for example, aspheric or multi-radial geometry contact lenses, glasses, corneal implants, orthokeratology (use of contact lenses to flatten the curvature of the anterior cornea), and various refractive surgical procedures, such as LASIK, PRK, and LASEK.
However, there are disadvantages associated with such conventional techniques. For example, with continued reference to FIG. 2, if the curvature of the field of focus 13 is flatter than the curvature of retina 15, a portion of the field of focus 13 is moved behind (posterior to) peripheral retina 17 when foveal myopia is treated, causing diminished image resolution in the retinal periphery. The eye responds to peripheral defocus by elongating the eye to cause the focal point of off-axis light rays to focus on or in front of the peripheral retina. Although the response of peripheral retina 17 achieves the desired effect of moving the focal point of the off-axis light in front of peripheral retina 17, it simultaneously causes the focal point of on-axis light rays to focus off fovea 16, thereby decreasing visual acuity. Moreover, correcting this defocus can initiate another round of peripheral defocus and eye elongation.
Specifically, research in the field of optics has recently established that poor image quality in the retinal periphery, in addition to the fovea, can play a major role in the development of myopia. When the peripheral retina does not sense clear focus (particularly when the field of focus is behind the retina), over time, the eye axially elongates in order to focus the off-axis light rays on or in front of the retina. In this regard, defocus at the peripheral retina has been demonstrated to be primarily responsible for the signal for axial length increase. The elongation of the eye increases the distance from the cornea to the retina, causing more of the on-axis rays to focus on or in front of the fovea. This effect is discussed, for example, in U.S. Pat. No. 7,025,460, Smith et al, entitled “Methods and Apparatuses for Altering Relative Curvature of Field and Positions of Peripheral, Off-Axis Focal Positions” (“Smith”).
An undesirable side effect of axial elongation is the focal point of on-axis light is moved off of the fovea, causing the previously-treated foveal myopia to reoccur. In other words, conventional techniques of treating foveal myopia can actually contribute to the progression of myopia in a patient over time.
In order to prevent axial elongation, the signal provided to the peripheral retina to stimulate growth must be minimized and/or eliminated by focusing off-axis light rays on or in front of the retina. To achieve this objective, Smith discloses optical, surgical and therapeutic systems and methods of altering the shape of the focal plane to keep the peripheral focal plane on or in front of the retina concomitant with methods of improving the focus of on-axis rays at the fovea. However, a disadvantage of the systems disclosed in Smith is that these methods alter the focal point of non-central on-axis light, such that it does not focus on the fovea. Since primary vision is dependent on light focusing on the fovea for precise acuity, it is undesirable to alter the focal point of non-central off axis away from the fovea.
As such, what is needed is a system and method capable of causing off-axis light rays to focus on or in front of the peripheral retina, without affecting the focal point of non-central on-axis rays.